Continuing pressure on the limited spectrum available for communication systems is forcing the development of spectrally-efficient linear modulation schemes. Since the envelopes of these linear modulation schemes fluctuate, intermodulation products can be generated in the non-linear power amplifier. Specifically in the private mobile radio (PMR) environment, restrictions on out-of-band emissions are severe (to the order of -60 to -70 dBc) and therefore linear modulation schemes will require highly linear transmitters.
The emphasis in portable PMR equipment is also to increase battery life. Hence, the operating efficiencies of the amplifiers used must be maximised. To achieve both linearity and efficiency, so called linearisation techniques must be employed in the power amplifier (P.A.), improving the linearity of the more efficient amplifier classes e.g. class AB, B or C amplifiers. One such linearising technique often used in designing linear transmitters at present is Cartesian Feedback. This is a "closed loop" negative feedback technique which sums the baseband feedback signal in it's "I" and "Q" formats to the "I" and "Q" input signals prior to amplifying and up-converting this signal to it's output frequency and power level.
A very useful and desirable implementation for trunked radio manufacturers would be for one radio unit to have the ability to operate in both trunked and direct/conventional modes. Copending patent application Ser. No. . . . entitled "Power Amplifier for Radio Transmitter and Dual Mode Remote Radio" filed on the same date as the present application suggests means for achieving this feature involving switching between closed and open loop modes.
One problem inherent in being able to switch between a negative feedback closed loop mode and an open loop mode is the variation in the resultant power output levels. Switching from closed loop operation to open loop operation causes the transmitter's total power gain to increase by a factor of the closed loop's "loop gain". Consequently the final P.A. device will be driven into saturation by an increase in its drive level of this "loop gain" and will not exhibit the desired 30 dB linearity performance required for it to meet the adjacent channel power levels for direct mode operation. The radio then causes too much interference in adjacent channels.
In certain linearising training schemes it is also necessary to switch between open and closed loop modes. Thus U.S. Pat. No. 5,066,923 of Motorola, Inc. describes a training scheme in which the phase of the amplifier is adjusted in an open loop mode, the loop is closed and the gain of the amplifier is adjusted. In that patent, the need to switch between equal open and closed loop gains is described and a method of achieving this is described in the form of selectively enabling and disabling a gain element in the loop. If a gain element is selectively enabled or disabled within the loop, this can have an impact on the phase shift and phase stability and on the complexity of the circuit.
It would be desirable to integrate a power amplifier and its loop control circuit into a single integrated circuit. Unfortunately, when a circuit of this nature is integrated, it is difficult to provide for adjustment of the gains of elements in the control loop. An improved circuit is needed which is more suited to integration.